This invention relates to laser beam scanning apparatus having a variable focal distance device of which the focal distance can be changed by electrical means, and the variable focal distance device for use in the apparatus; and particularly to a laser beam scanning apparatus having a variable focal distance device which can substantially change the diameter of the scanning spot in one-dimensional direction.
The conventional laser beam scanning apparatus, such as a laser printer, changes the focal distance of the laser beam scanning lens (for example, an f.theta. lens) by mechanically changing the distance(s) between the combined lenses which constitute the scanning lens, or by inserting another optical device therebetween.
This mechanical means has, in effect, a difficulty in changing the focal distance in less than one ms (millisecond). However, the technique for changing the focal distance at high speed has been demanded in the optical disc apparatus for recording multilevel information, and in the laser printer for halftone printing.
An example of this technique for changing the focal distance at high speed is described in an article in Tech Digest of Conference on Laser and Electro-Optical Systems, "Optical System of an Ultra-high speed Laser Beam Printer", Arimoto et al., CLEOS'80, p. 76, WII2 (1980).